Is there a theoretical maximum for intelligence?
Note that the statement says nothing directly about the limit of intelligence, nor even about the limit of computational intelligence - but about the limit of computing power.

Perhaps the sentence "the speed of light puts a strict upper bound on how much computing can be done" needs a better explanation: 

The authors are probably referring to Bremermann's limit, which defines an upper bound in bits per second per kilogram. It is an upper bound on the processing power per unit of time of a computer with a given weight.

There is also Margolus–Levitin theorem which defines an upper limit in operations per second per joule. It is an upper bound on the processing power per unit of energy.

These principles do not define a theoretical limit on computing power, but a practical one. If you'll limit your computer and your energy source to the size and capacity of the earth (or to the those of the universe) - you'll get a very practical limit.

Check the reference section in Wikipedia article Limits to computation